1. Field of the Invention
The present invention relates to apparatus for measuring insulation leakage in motors and, more particularly, to apparatus for monitoring the windings of an in-service electrical pump motor and the like for leakage and for inhibiting operation of the motor when excessive leakage is indicated.
2. Description of the Prior Art
In electrical systems which utilize electric motors, the insulation of the motor windings can deteriorate, causing leakage of current to ground. High leakage currents can cause damage to the motor windings requiring replacement or rebuilding of the motors. In particular, systems utilizing submerged pump motors, such as lift stations, wells and other water systems, are particularly susceptible to insulation damage and leakage. In a submerged motor, it is common to utilize double shaft seals to prevent water or moisture from getting into and coming in contact with the motor windings. Under normal use, these seals may deteriorate permitting a resultant leakage of water. If the problem is caught in a timely fashion, the motor may be removed, disassembled and baked out to remove moisture which may have infiltrated the windings. The seals may be replaced, and the motor may be reassembled and placed back in service. The cost for such repairs is minimal compared to the cost if the deterioration were allowed to continue to the point of complete failure of the motor windings.
To forestall serious difficulties, a usual practice is to periodically test the insulation resistance from the windings to neutral or the ground connection to the motor frame. This is done with a megger, which may be a hand cranked magneto device that places several hundred volts between the winding and the motor frame and measures any leakage currents. If the leakage exceeds a predetermined value, it is an indication that there is an incipient seal failure. This manual approach to testing is time consuming where numerous pumps and motors are in use. In many instances, such testing may be done so infrequently that failure can occur before scheduled testing.
There is a need for apparatus which can provide continuous monitoring of leakage resistances in motors so that potential problems may be recognized long before serious damage is done. One approach to this need is disclosed in U.S. Pat. No. 4,159,501 to White, which teaches the use of a high voltage direct current power source connected to an electrical motor to be monitored. The apparatus provides an output indication in response to a first leakage resistance and another indication responsive to a second leakage resistance, producing a signal warning in response to the second resistance value. In addition, an interrupter is provided to remove primary power from the load when leakage is detected. A disadvantage of the White apparatus is that the direct current high voltage supply, which may be 500 volts or so, is connected to the power line at all times even while the motor is in operation. Thus, not only will the leakage resistance of the motor to ground be measured, but also that of every other load, such as other motors and other electrical devices on the same circuit. In addition, the high voltage is added to the ac voltage present which will cause very large peaks of high voltage which can result in arcing and/or damage to insulation.
A monitoring device for motor winding insulation resistance is disclosed in Japanese Patent No. 84,370 to Teraguchi. Teraguchi shows a field winding having a dc power supply connected to the motor, a leak detector, an indicator, and a switch, but has no means disclosed for automatically monitoring the leakage resistance. U.S. Pat. No. 4,540,922 issued to the present inventors and assigned to Automeg, Inc. discloses an apparatus for automatically monitoring motor winding leakage in dual motor installations.
There exists a need for a simple low cost insulation resistance monitor for a single motor and the like which will be automatically connected to the motor windings only when the motor is not in operation which will give a positive indication when leakage resistance becomes excessive and, in such instance, inhibit operation of the motor.